Deposition of thin films on a substrate surface is an important process in a variety of industries, including semiconductor processing, diffusion barrier coatings, dielectrics for magnetic read/write heads, and random access memory. In contrast to volatile types of memory, such as DRAM memory modules, resistive random access memory (“ReRAM” or “RRAM”) has gained much interest as one of the possible candidates of future non-volatile memory devices which can replace conventional flash memory because of its simple structure, long retention time, high switching speed, and high scalability. Metal oxide films incorporating transition metals are used in semiconductor applications including high K gate dielectric films, active materials for ferroelectric memories, thin film battery cathodes, materials in silicon based light emitting devices and memory cells. RRAM is an emerging memory type that relies on oxygen filaments in the metal oxide film to modulate the resistance of the memory cell. Oxygen deficient metal oxides are desirable for the operation of the RRAM cell. However, the ability to control the specific composition of the metal oxide deposited layer can be limited. Many metal-oxygen condensed phase systems employ metal oxides that are known to be stable at different oxidation potentials and have well-defined stoichiometric phases. For these materials it is generally possible to consistently obtain a desired metal oxide once an oxidation potential threshold is exceeded, and equilibrium is reached. However, for applications in which metal oxide films deficient in oxygen are desired, processes such as “scavenging” or “gettering” to absorb some of the oxygen have been employed once the stoichiometric deposition has occurred by physical vapor deposition (PVD) or chemical vapor deposition (CVD).
PVD has the limitation of being a non-conformal deposition method and has issues for extendibility to three-dimensional memory architectures. With atomic layer deposition (ALD), a variation of CVD, it has been virtually impossible to deposit oxygen deficient metal oxide films because ALD is a self-limiting, saturated process by definition. There is therefore a need for CVD and ALD processes in which oxygen levels can be better controlled to produce transition metal oxide thin films on substrate surfaces which have oxygen deficient compositions. This need is particularly acute in the field of deposition of nonstoichiometric metal oxides, such as transition metal oxides.
In RRAM, oxygen vacancies are needed to carry electrical charge through the dielectric layer to conduct electric signal between the top and bottom electrodes. Therefore it is desirable to be able to control the metal to oxygen ratio (M:O) in metal oxides MOx.